1. Technical Field
This invention relates to systems for removing particulates from the exhaust emissions of an internal combustion engine (particularly a diesel engine) and, more particularly, to the use of cooling to promote the removal of both hydrocarbons and carbon particulates from such emissions.
2. Description of the Prior Art
Certain benefits of cooling the exhaust gas downstream of the engine but upstream of a conventional particulate trap have been recognized. It facilitates the cleaning of exhaust gases as disclosed in U.S. Pat. No. 4,319,453. In this reference, the cooling of the exhaust gas by an air to air heat exchanger reduces the exhaust gas temperature to below 150.degree. F. (120.degree.-150.degree. F.) and condenses out substantially all of the heavy hydrocarbons contained as a gaseous emission in the exhaust gas; such hydrocarbons are absorbed by the carbon particulates also contained in such exhaust gas. Condensation of substantially all of the hydrocarbons will unduly saturate the carbon particles, inhibiting the lowering of ignition temperature of the carbon particles. Maximization of such hydrocarbon condensation is desired by the authors of this reference in order to be able to fill a disposable filter. Unfortunately, carbon particulates will load up a trap at a rate of 20 grams per 80 miles of vehicle operation, necessitating disposal of the trap too frequently. If on-board trapping of particulates and precipitates is to avoid trap disposal, then the collected particles must be periodically combusted and reduced to harmless gases and released to atmosphere. Thus, trap regeneration is a necessity.
Cooling of exhaust gases is also disclosed in U.S. Pat. No. 3,100,146, wherein a smog reducing device utilizes both gas to gas and gas to liquid heat exchangers to insure a.steep drop in the exhaust gas temperature to promote condensation of substantially all of the partially burned and oxygenated hydrocarbon vapors. Since this patent does not relate to diesel engines, the carbon particulate content is low, forcing the hydrocarbons to agglomerate independently by themselves to produce saturated globules. Such globules are attracted to an impingement filter, of the disposable kind, without regeneration. Thus, this reference does not direct itself to the problem of utilizing cooling to facilitate regeneration.
With the advent of adiabatic diesel engines wherein engine cooling may be entirely eliminated, there will be no engine waste heat available for heating a vehicle cabin. Extraction of heat after it enters the exhaust system would be welcomed for use to heat the vehicle cabin. The prior art has only dealt with cooling of exhaust for engines that already have their own engine cooling system. The above discussed prior art deals with a contemplated exhaust gas temperature drop of only 100.degree.-180.degree. F. for conventional internal combustion engine use for passenger vehicles where the exhaust gas temperature is no higher than 300.degree.-400.degree. F. and the extraction of heat from such exhaust gas temperature results in a comparatively small temperature drop and heat content extraction. Exhaust gas temperatures from an adiabatic engine will be much higher due to the absence of an engine cooling system and due to the presence of insulating materials retaining the heat of the engine. To substantially cool such high exhaust gas to a temperature below 200.degree.-150.degree. F. in accordance with the teachings of the two above-mentioned references would not be economically feasible except under certain unusual engine operating conditions. The exhaust gas temperature can be dropped 30-60%, however, by use of gas/liquid heat exchangers resulting in temperature drops of up to 500.degree.-600.degree. F. with the exhaust gas temperature still remaining above 200.degree. F. This results in only partial condensation of hydrocarbons, but the heat extraction content is significant. Such heat content can be used for cabin heating, etc.
The controls for regeneration present still another problem associated with extraction of carbon particulates of diesel engines. Heretofore, the prior art has progressed in the direction of greater control complexity, such as by the use of computers to digest a variety of operating parameters for deciding when to establish regeneration and for how long (see U.S. Pat. Nos. 4,544,388; 4,492,079; and 4,567,725). It would be desirable if such controls could be reduced in complexity without affecting reliability.